The processing of signals from semiconductor detectors can be effected in high-resolution spectroscopic analysis systems as an optimization compromise to determine with the greatest possible precision, the energy of the radiation quanta (X-ray radiation, gamma radiation, charged-particle radiation) at the greatest possible throughput In gamma-ray spectroscopy, for example, because of the high intrinsic resolution (about 0.1%) of the germanium detector the analog detector signals have to be analyzed with an integral non-linearity of better than 0.02% and a differential nonlinearity of better than 0.3% over a dynamic range of about 70 dB. The targeted throughput or rate is 10.sup.4 -10.sup.5 events per second.
The range of applications of semiconductor spectroscopy systems is very wide and ranges from measuring systems for environmental protection, medicine and materials research to large scale experimental projects in nuclear physics and cosmology basic research
In conventional techniques for the processing of semiconductor signals, it has been common heretofore to make use of analog pulse amplifiers (spectroscopy amplifiers SA) in combination with high-resolution analog-to-digital converters (ADC). Within the spectroscopy amplifier a pulse shaping of the preamplifier signal is performed with the goal of optimizing the throughput and the signal/noise ratio to yield an output signal whose amplitude corresponded with the greatest possible precision to the charge generated in the semiconductor detector Additionally, the spectroscopy amplifier usually contains circuitry for zero-line stabilization (baseline restoration) and for elimination of pulse pile-ups (pile-up rejection). The output signal of the spectroscopy amplifier is digitalized by the ADC. The ADC contains, in addition to the peak detector/stretcher and a linear gate, to fix the signal amplitude during the conversion interval, an amplitude-to-time converter and, finally, a time-to-digital converter (Wilkinson ADC).
This conventional type of signal processing has a number of drawbacks The effective resolution of the semiconductor spectrometer is limited by the "ballistic deficit" and the "charge trapping" effects. The maximum throughput is limited by the pulse pile-up and the long conversion time of the ADC's. Finally the system has a high degree of sensitivity to temperature fluctuations and poor long-term stability because of the large number of analog components used in each signal channel.
The "ballistic deficit" effect is caused by differences in the charge distribution of the detector signal in the charge-collection interval and manifests itself in different shapes and rise times of the leading edge of the preamplifier output signal. The fixed transfer function of the preadjusted pulse-shaping network, in the spectroscopy amplifier SA compensates only partly these differences and transforms them into fluctuations of the pulse amplitude in the direction of decreasing values. It is this reduction in the pulse amplitude which is referred to as the "ballistic deficit." Recently various methods based upon analog circuitry has been developed to correct for these deficits in the pulse amplitude, as well as for the deficits caused by "charge carrier trapping" effects, (F. S. Goulding and D. A. Landis, IEEE 35 (1988) p. 119; M. L. Simpson et al, IEEE 36 (1989) p. 260; S. M. Hinshaw and D. A. Landis, IEEE 37 (1990) P. 374). The most common correction technique was the Goulding-Landis method.
The latter determines the correction factor based upon different time shifts of the pulse maxima after pulse shaping. It yields some improvement in the energy resolution, but nevertheless a significant energy dependency of the result remains and it is not able to account for minority charge carrier trapping effects.
In order to optimize the throughput of spectroscopy analyzer system, attempts have been made in both directions, namely on the one hand, to minimize the dead time of the analog spectroscopy amplifier, and, on the other hand, to minimize the conversion time of the analog-to-digital converter.
To reduce the dead time of the analog signal processing, a gated integrator (GI) has been introduced. In the gated integrator system, a pulse-shaping network with a comparatively small time constant is followed by the gated integrator. With this system, it is possible to obtain an improved behavior of the spectroscopy amplifier at high counting rates, but at the expense of a overall poorer energy resolution.
A reduction in the conversion time of the analog-to-digital converters (ADCs) can be achieved by replacing the Wilkinson method by a binary weighting (successive approximation) method (P. Casoli and P. Maranosi, Nucl. Instr. Meth. 166 (1979) p. 299). Since the last method has a significant inherent differential nonlinearity it must be combined with a sliding scale method (E. Gatti, Nucl. Instr. Meth. 24 (1963) p. 241). The combined method yields a significant reduction in the conversion time, but nevertheless has several drawbacks. One is, like with the Wilkinson method, the need of analog circuitry like peak-detector/stretcher and linear gate in order to fix the pulse amplitude for the conversion interval. This again gives rise to various drawbacks sensitivity with respect to count rate and temperature fluctuations. A second is that the advantage of the sliding scale method, which in principle is an averaging method requiring a statistical significant number of measured values is lost because in the case of low statistic or individual measurements due to the bad channel profile.
In general, practically all currently used methods for processing semiconductor detector signals have temperature and long-term instabilities which can have a significant effect upon the main system parameters like resolution and integral linearity. This can be mainly attributed to the large number of analog electronic components which one finds in conventional spectroscopic analyzer systems and to their sensitivity to external influences.
Recently efforts have been made to overcome the problem of the stability of spectrometer systems by replacing the gated integrator and the analog-to-digital converter (ADC) with a fast digitalizer (flash ADC). In this method, the signal shape is digitalized after the analog pulse shaping. This technique also results in incomplete handling of the problems of "ballistic and charge carrier trapping" deficits, since these problems have their origin in the analog pulse shaping. In addition the method introduces a new, special "ballistic deficit" problem, whose origin is to be found in the uncorrelated time position of pulse maxima relative to the sampling raster. As a consequence, this method has to date found application only in cases in which a high energy resolution is not required.